β-BaCO ₃ Facilitates a Topochemical Reaction Route for the High-Temperature Synthesis of Perovskite-Type (Ba,Ca)(Zr,Ti)O ₃ via Oxycarbonate Intermediates

High Resolution Image Download MS PowerPoint Slide Ferroelectric oxides from the perovskite-type (Ba,Ca)(Zr,Ti)O 3 system are emerging as one of the upcoming lead-free piezoelectrics for future sensing and actuating applications. Improving their functionality requires a precise adjustment of precursor choice, pretreatment, as well as reaction and sintering times and temperatures to optimize the defect chemistry and microstructure of the material. In this respect, understanding the reaction pathways of the precursors has a crucial impact on the final functional properties. In this article, we show that the high-temperature modification of the BaCO 3 precursor can strongly influence the reaction by facilitating a topochemical reaction pathway via the formation of an intermediate oxycarbonate phase of BaTiO 3– x (CO 3 ) x ( x ≈ 1/3). This results from the diffusion of Ti 4+ and 2 O 2– into the material in combination with CO 2 outgassing, distinguishing this reaction path from the previously reported diffusion mechanism via an intermediate phase of Ba 2 TiO 4, which happens in parallel. We used high-temperature in situ X-ray diffraction and automated diffraction tomography to derive a structural model of this previously unknown intermediate oxycarbonate phase, which can be understood as a trigonal distortion variant of the cubic perovskite structure, where the trigonal distortion originates from the planar nature of the carbonate anion. This mechanism might have implications on how the BaCO 3 precursor and its decomposition behavior could decisively influence particle morphology and composition obtained prior to sintering of (Ba,Ca)(Zr,Ti)O 3 ferroelectric oxides.